1. Field of the Invention
The present invention generally relates to long distance telephone communication systems, and more particularly to the application of frequency-selective center clipping in echo suppressors for use in such communication systems.
2. Description of the Prior Art
In long distance telephone communications transmissions in opposite directions are carried over physically separated cable pairs. This separation of direction is called four-wire transmission. It will be appreciated that a four-wire network broadly defines separate send and receive paths, and these paths may be wireless, such as up and down links to a satellite. A commercial telephone instrument transmits and receives over the same pair of wires. Although such two-wire transmission is entirely satisfactory for local telephone calls, for long distance connections it is necessary to convert from two-wire transmission to four-wire transmission. This is typically accomplished by a hybrid transformer circuit. To prevent energy in the receive path from entering the send path, the net impedance and the impedance seen on the telephone side of the hybrid circuit must be very closely matched. The latter, however, varies from one telephone connection to another due to variation in the distances from subscriber' telephones to the hybrid circuit. Thus, a compromise net impedance can realize an average of only about 12 dB separation with a standard deviation of 3 dB between receive and transmit sides.
Due to the impedance imperfections in the conversion from two-wire transmission to four-wire transmission by the hybrid, some fraction of the signal energy originating from a source at one end is reflected at the other end. This is perceived as echo to the person who originated the signal and, due to the propogation time of the four-wire circuit in long-distance connections, the echo becomes very noticeable. This is an undesirable phenomenon and, as a result, voice-energy switched devices known as echo suppressors have been devised to prevent echo from occuring. Echo suppressors are placed in the four-wire part of the long distance circuit, one at each end. There are inherent limitations in the performance of circuits equipped with echo suppressors due to the design approach of voice switching. These limitations are particularly noticeable when both talkers speak simultaneously.
The basic principle of operation of most echo suppressors is based on blocking the echo signal by speech switching. This blocking of the echo signal is typically accomplished by a receive speech detector-amplifier whose output controls the insertion of a high loss in the transmit path. The major drawback of this technique is that it is applied to the full possible range of the signal amplitudes, even though echo signals are generally restricted well below that range because of the finite value of the echo return loss.